1. Field of the Invention
The present invention relates to stable aqueous enzyme compositions and, more particularly, to such compositions for use in heavy-duty liquid detergents.
2. Description of the Prior Art
Heavy-duty liquid detergents (HDLs) are rapidly becoming the detergents of choice. Many of these HDLs are formulated using enzymes, particularly proteolytic enzymes. The use of proteolytic enzymes in HDLs is complicated by their limited stability in aqueous solutions, particularly when the HDLs contain an anionic surfactant, such as, for example, a linear alkylbenzene sulfonate (LAS), an alcohol sulfate (AS), a secondary alkane sulfonate (SAS), an alpha olefin sulfonate (AOS), etc. These and other anionic surfactants are known to destabilize commercial proteolytic enzymes (proteases). In particular, since LAS has become the most widely used anionic surfactant, the enzyme compatibility problem in liquid formulations has become synonymous with the use of LAS.
In order to effectively use LAS or other anionic surfactants in HDLs, it has become common practice to use stabilizers. Examples of stabilized compositions of proteolytic enzymes are disclosed in U.S. Pat. Nos. 5,071,586; 4,305,837; and 4,318,818, to mention a few. Thus, U.S. Pat. No. 5,071,586 discloses a stabilized composition that employs propionic acid or a propionic acid salt capable of forming propionic acid, the composition being built and having a pH greater than 8.5, preferably 9.0 and above. U.S. Pat. No. 4,305,837 discloses the use of a low molecular weight primary or secondary alcohol together with a short-chain-length carboxylic acid salt, preferably a formate as a stabilizer, while U.S. Pat. No. 4,318,818 uses a similar stabilizing system.
As is typified by the above patents, the solution to stabilizing HDL-containing enzymes, particularly proteolytic enzymes, has been to add compounds that, for the most part, serve no useful function other than to act as a stabilizer or inhibitor to prevent loss of activity of the enzyme. Thus, while these inhibitor or stabilizing systems are effective for their intended purpose, they add cost to the composition without imparting any beneficial effect other than to prevent loss of activity of the enzyme.
It would clearly be desirable to have stable aqueous enzyme compositions wherein the stabilizing agent or compound, over and above preventing loss of activity of the enzyme, contributed to the efficiency of the composition, e.g., exhibited surfactant properties.